This invention relates to the field of fluid separation membranes and materials useful therein, especially to a polyolefin coated with a highly selective fluoropolymer.
In creating a gas or liquid separation membrane, it is desirable to have both a high rate of permeation or throughput and a high separation factor. This combination of characteristics permits the effective separation of a relatively large volume of fluid per unit time. Large rates of permeation also allow the membrane to be operated at lower pressures, which improves cost-effectiveness.
Highly porous membranes tend to be very permeable, but do a poor job of separating fluids into their components. Less porous, dense membranes can be more selective, but at the cost of reduced throughput.
Asymmetric membranes generally have a thin, dense separation layer and a less dense, more permeable layer. These asymmetric membranes are often chosen for separation applications because they may provide a good combination of permeation and separation. Hollow fibers having a dense skin and a porous body can be used to make such membranes.
Unfortunately, many of the highly selective membrane materials, e.g. fluoropolymers, are relatively expensive. Some processes are also expensive, particularly if many process steps are needed. Furthermore, in some fibers the permeability, the separation factor, or both may be rather low. One solution is to use a porous, inexpensive material, such as polypropylene, and coat it with a thin layer of a fluoropolymer or other selective material. However, materials such as polypropylene may not adhere well to the selective material.
U.S. Pat. No. 4,230,463 issued to Henis, et al. describes multi-component membranes for gas separations wherein a porous membrane is coated with another substance. The coating is said to enhance the separation capability of the membrane.
U.S. Pat. No. 4,756,932 issued to Puri describes a process for making highly permeable coated composite hollow fiber membranes. The hollow fiber substrate is passed continuously through a polymeric coating solution and withdrawn through a coating die to form a uniform coating.
U.S. Pat. No. 4,881,954 issued to Bikson, et al. describes permeable membranes for enhanced gas separation in which an asymmetric porous support is coated with a separation layer. The support is made asymmetric prior to coating to provide mechanical stability and a uniform support surface.
U.S. patent application Ser. No. 738,996 filed on Aug. 1, 1991 by Chung, et al. describes an asymmetric hollow fiber consisting essentially of SIXEF.TM.-Durene, a polyimide made from 2,2-bis[3,4-dicarboxyphenyl] hexafluoropropane dianhydride and 2,3,5,6-tetramethylphenylene diamine monomers.
The disclosures of the foregoing references are incorporated by reference herein.